The present invention relates to curing of isocyanates in the presence of quaternary ammonium catalysts and more particularly for a unique method for producing such catalyst in situ.
Quaternary ammonium salts are known to be efficient trimerization catalysts and are known to be effective in making isocyanurate-type polyisocyanates. Trimerization of isocyanates into isocyanurates is a reaction that can be continued for formation of coatings, adhesives, elastomers and plastics, and like products. The isocyanate trimerization reaction, and resulting cure, can be represented as follows: ##STR1## The trimerization reaction usually requires some heat even in the presence of quaternary ammonium catalysts. The predominant form of the quaternary ammonium catalyst comprises quaternary ammonium hydroxide salts as proposed by German Pat. No. 1,150,080.
U.S. Pat. No. 3,211.703 proposes to make quaternary ammonium hydroxide catalysts for isocyanate trimerization by reacting a tertiary amine and an epoxide. U.S. Pat. No. 3,919,218 proposes the use of aziridine and a tertiary amine co-catalyst that can be destroyed by heat in the trimerization reaction. U.S. Pat. No. 3,487,080 proposes trimerization of isocyanates in the presence of quaternary ammonium hydroxides plus a compound selected from a phenol, an oxime, or methanol. U.S. Pat. No. 4,040,992 conducts the trimerization reaction utilizing shortened carboxylic acid salts of quaternary ammonium hydroxyl compounds. U.S. Pat. No. 4,288,586 proposes the use of quaternary hydroxyalkyl ammonium hydroxides containing at least one hydroxyalkyl group in the polymerization of isophorone diisocyanate. U.S. Pat. No. 4,324,879 proposes the use of quaternary ammonium hydroxyl catalysts with the hydroxy group of the hydroxyalkyl substituent at the two position with respect to the nitrogen atom. U.S. Pat. No. 4,560,494 also shows the use of quaternary ammonium hydroxyl salts in isocyanurate formation. Finally, general synthesis schemes for forming quaternary ammonium hydroxyl salts also can be found in U.S. Pats. Nos. 3,010,963 and 3,995,997.
It is well documented that tertiary amines react with epoxy groups to form a Zwitterionic species which is in equilibrium with its two reaction components. While such zwitterionic species probably is an effective trimerization catalyst, formation of significant catalytic quantities thereof requires large amounts of reactants because of the equilibrium involved. If the zwitterionic species could be stabilized, e.g. through a non-equilibrium reaction, small quantities of reactants then could be used for producing effective trimerization catalysts.